Conventional coal-fired combustion devices produce emissions that include pollutants such as mercury. Mercury vapor is a poison of the nervous system, with chronic mercury poisoning having potentially dire consequences. Mercury poisoning can at times be fatal and has the characteristic of being cumulative over years of exposure, as the body's nervous system has difficulty in purging this element. At the levels common in the atmosphere, the concentrations of mercury are usually safe. However, mercury can accumulate in lakes, rivers, streams, or the like as a result of rainfall. The mercury can then be ingested by fish, ducks, and other wildlife. This wildlife can be destroyed by the mercury poisoning, or the wildlife, with organic mercury molecules in them, can be hazardous to individuals who eat them. Some conventional systems attempt to control mercury emissions with particulate collection devices.
Mercury (elemental symbol Hg) is a metal that melts at 234K (−38° F.) and boils at 630K (674° F.). As such, it can be expected to have a high vapor pressure relative to many metals. However, the oxidized forms Hg++ and Hg+ have much lower vapor pressures and can be captured by flyash particulates. It is much easier to collect the oxidized forms that are attached to particulates with conventional particulate collecting devices than it is to collect elemental mercury (Hg), which can be in its vapor or gaseous form at flue gas temperatures.
Accordingly, some conventional systems inject additives into flue gas to oxidize the mercury prior to collection. However, baghouses, fabric filters, electrostatic precipitators, and other collection devices that are efficient enough to reduce the mercury emissions to levels that may be required are very expensive. Moreover, it is still possible for elemental mercury vapor to escape as a gaseous vapor molecule.
Still other conventional systems utilize activated carbon and other fine particulates to bind or absorb mercury to facilitate removal of oxidized mercury. However, the efficiency of electrostatic precipitators may be diminished by high carbon content in fly ash, which results from the use of activated carbon, and thus baghouses are required to remove carbon-containing particles before the flue gas enters the electrostatic precipitator. Also, flyash having a high carbon content is not sellable, and therefore presents a disposal problem.
Since several states and the United States Environmental Protection Agency will soon limit the emissions of mercury from combustion devices, efficient and cost-effective apparatuses and methods for controlling emissions of mercury are desirable.